For the first time, scientists have used genetically modified viruses to treat a patient fighting an antibiotic-resistant infection.

Isabelle Carnell-Holdaway, 17, began the experimental treatment after doctors lost all hope. She was struggling with a life-threatening infection after a lung transplant. With the new treatment, she has not been completely cured. But the Faversham, England, teenager has recovered so much that she has resumed a near-normal life.

"I think it's amazing," Carnell-Holdaway says. "It kind of shows that there is completely no limit to what they can come up with really."

Doctors and scientists stress that it's difficult to draw firm conclusions from one patient, whose experience is described in an article in the journal Nature Medicine published Wednesday. But they hope the case will spur more efforts to try the approach on more "superbug" infections and help counter the growing threat posed by drug-resistant infections.

"This is actually a historic moment," says Steffanie Strathdee, a professor of medicine at the University of California, San Diego. She co-directs a center that focuses on phages, which are viruses that infect bacteria.

"This is the first time that a genetically engineered phage has been used to successfully treat a superbug infection in a human being," Strathdee says. "It's terribly exciting."

A phage, shorthand for bacteriophage, is a natural enemy of bacteria. Phages have been used for decades to try to treat infections. But the organisms have not been extensively studied or incorporated into general medical use in most countries.

Phages have, however, been used periodically to treat individual patients when no other options are available. Strathdee, for example, wrote the book The Perfect Predator, a dramatic story about how a last-ditch phage treatment apparently saved her husband.

When she was 11 months old, Carnell-Holdaway was diagnosed with cystic fibrosis, a genetic lung disease. She underwent a double lung transplant at the Great Ormond Street Hospital in London in September 2017, when she was 15.

As she was recovering from the transplant, a preexisting chronic infection quickly spread throughout her body. The antibiotic-resistant bacteria infected her liver, lungs and surgical wound, and caused skin lesions all over her body. All treatments failed.

"We've transplanted a number of these patients, and once they get a recurrence of infection in this way, all of those patients have gone on and died," says Helen Spencer, Carnell-Holdaway's doctor.

Spencer and her colleagues told Carnell-Holdaway's family there was nothing more they could do to save the teenager.

"We were devastated," says her mother, Joanne.

But at Joanne's urging, Spencer and her colleagues contacted Graham Hatfull, a professor of biological sciences at the University of Pittsburgh who specializes in phage research.

Hatfull searched a collection of 15,000 phages he has assembled with his students. He identified one that appeared to be good at killing the bacterium, Mycobacterium abscessus, which was causing the girl's infection. (As it turns out, the phage he identified, known as "Muddy," was discovered in a rotting eggplant.)

Hatfull's team also identified two other phages that could be useful, which they then genetically modified in a way they hoped would boost their ability to zero in on and kill the bacterium attacking the teen.

"Using genetic approaches with genome engineering, we were able to assemble this collection of three phages that we could then combine in a cocktail to use for treatment. They not only infect, but kill efficiently," Hatfull says.